A laser angular sensor consists primarily of first and second laser beams which are propagated in opposite directions around a path enclosing a rotational sensing axis about which the sensor is to detect and determine angular change. Rotation of the apparatus about that axis changes the effective path length that each beam must travel. The detected frequency difference between the two laser beams is a measure of the angular rate or velocity of the sensor about its sensing axis. The polarity and magnitude of detected frequency change are measures of the direction and magnitude of physical rotation being sensed.
At low rates of rotation of each sensor about its sensing axis, the detected difference in beam frequency becomes disproportional to the rotational rate, and the two counterpropagating laser beams exhibit a single frequency. That condition of no frequency difference in the presence of a small angular rate is called "lock-in".
One apparatus for avoiding the effects of lock-in mechanically oscillates or dithers each laser sensor about its sensing axis. The dither motion is designed to minimize lock-in error and to make each instrument sensitive to small angular velocities.
For navigation of an aircraft, for example, three laser angular sensors are typically arranged with their sensing axes orthogonal to each other and mounted relative to the aircraft on a common platform or instrument block. By sensing the rotational change and rate of change about each of the three orthogonally related axes, angular movement or change in orientation of the vehicle from an initial direction can continuously be determined. To avoid lock-in, each laser angular sensor is supported by springs to allow dither about its sensing axis. Typically each laser receives controlled dithering energy. When the three sensors are mounted on a common instrument block or rigid structure, the individual dithering motions of the sensors are mechanically coupled through the elasticity of the instrument block or structure and their supporting springs. When the instruments are dithered at the same mechanical frequency with appropriate phase angles between the dithering of the three sensors, the mechanical coupling causes the supporting structure or instrument block to wobble, and the wobbling is likely to produce dither-induced coning motion. Because of the misalignment of the instrument sensing axes caused by the coning, significant errors in the navigation system information can occur. Typically, the navigation system delivers earth-position and/or ground velocity of the supporting vehicle, and the delivered information may be in error. Attempts have been made to correct or compensate for coning by using particular circuits in which the coning is sensed, and the output signals from the different laser angular sensors are modified. However, such attempts have not been completely satisfactory.
One such attempt is disclosed in U.S. Pat. No. 4,277,173 which relates to a technique for eliminating frequency and phase differences in the three dither motions.